NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, selective catalytic reduction (SCR), and lean NOx traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
LNTs are NOx adsorbants with catalysts that reduce NOx during regeneration. The adsorbant is typically an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst is typically a precious metal, such as Pt or Ru. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. Accumulated NOx is removed and the LNT is regenerated by creating a reducing environment within the LNT. In a rich environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed.
Regeneration to remove accumulated NOx may be referred to as denitration in order to distinguish desulfation, described below. The reducing environment for denitration can be created in several ways. One approach uses the engine to create a rich fuel-air mixture. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. A reducing environment can also be created by injecting a reductant into the exhaust downstream of the engine. In either case, a portion of the reductant is generally expended to consume excess oxygen in the exhaust. To lessen the amount of excess oxygen and reduce the amount of reductant expended consuming excess oxygen, the engine may be throttled, although such throttling may have an adverse effect on the performance of some engines.
Reductant can consume excess oxygen by either combustion or reforming reactions. Typically, the reactions take place upstream of the LNT over an oxidation catalyst or in a reformer. The reductant can also be oxidized directly in the LNT, but this tends to result in faster thermal aging. As an example, U.S. Pat. Pub. No. 2003/0101713 describes an exhaust system with a fuel reformer placed inline with the exhaust and upstream of a LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate.
In addition to accumulating NOx, LNTs accumulate SOx. SOx is the combustion product of sulfur present in ordinarily diesel fuel. Even with reduced sulfur fuels, the amount of SOx produced by diesel combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere. The elevated temperatures required for desulfation can be produced by oxidizing reductant.
It is known that a NOx adsorber-catalyst can produce ammonia during denitration and from this knowledge it has been proposed to combine a NOx adsorber-catalyst and an ammonia SCR catalyst into one system. Ammonia produced by the NOx adsorber-catalyst during regeneration is captured by the SCR catalyst for subsequent use in reducing NOx, thereby improving conversion efficiency over a stand-alone NOx adsorber-catalyst with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 describes such a system. U.S. Pat. Pub. No. 2004/0076565 describes such systems wherein both components are contained within a single shell or disbursed over one substrate. WO 2004/090296 describes such a system wherein there is an inline reformer upstream of the NOx adsorber-catalyst and the SCR catalyst.
It is known that LNTs function optimally only within limited temperature ranges. U.S. Pat. Pub. No. 2003/0074888 states that NOx reduction by a LNT is particularly efficient in the temperature range from 300 to 350° C. The disclosure suggests heat exchange within the exhaust gas treatment system to maintain temperatures within a desired range. U.S. Pat. No. 5,404,719 suggests another method of maintaining the temperature of a LNT within a range where adsorption is efficient. When the temperature needs to be increased, fuel is injected into the exhaust. When the temperature needs to be decreased, air is injected into the exhaust.
U.S. Pat. No. 6,866,610 suggests using a continuously variable transmission (CVT) to prevent a catalytic converter having a NOx storage reduction catalyst from cooling below an activation temperature. In general, the CVT system is controlled to provide torque multipliers at which the engine produces a required power with optimal fuel economy. If, however, the optimal fuel economy operating point would place the exhaust temperature in a low range, a different torque ratio and engine operating point is selected to increase the exhaust temperature.
Some other uses of a CVT in connection with exhaust aftertreatment have been proposed. U.S. Pat. No. 6,135,917 describes using CVT to select operating points to speed the light-off of a catalytic converter. U.S. Pat. No. 6,157,885 describes using a CVT system to avoid high exhaust temperatures that would damage an exhaust gas purification system. U.S. Pat. No. 6,188,944 suggests using CVT to mitigate torque variations when a lean-burn gasoline engine is run rich in order to regenerate a LNT.
In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and is practical for reducing NOx emissions from diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.